1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer and to a developer cartridge mounted in the image forming apparatus.
2. Description of the Related Art
As a configuration for detecting the residual amount of toner in an image forming apparatus such as a laser printer, there has been heretofore known a light transmission type detection technique in which a photo interrupter having a light-emitting element and a photo acceptance element provided opposite to the light-emitting element is used for performing light irradiation to detect whether toner is present or not (e.g. see JP-A-7-219412) For example, in this technique, window holes are provided in opposite surfaces of a casing in which a developer is stored. Light-transmissive window members are fitted into the window holes respectively. A light-emitting element is provided in the outside of one of the window holes, whereas a photo acceptance element is provided in the outside of the other window hole. Light is transmitted through the casing, so that a judgment is made on the basis of the output value of the photo acceptance element as to whether toner is present or not.